Triplet anthraquinone

Oxidative repair is not a unique feature of our Rh(III) complexes. We also demonstrated efficient long-range repair using a covalently tethered naphthalene diimide intercalator (li /0 1.9 V vs NHE) [151]. An intercalated ethidium derivative was ineffective at dimer repair, consistent with the fact that the reduction potential of Et is significantly below the potential of the dimer. Thymine dimer repair by a series of anthraquinone derivatives was also evaluated [151]. Despite the fact that the excited triplets are of sufficient potential to oxidize the thymine dimer ( 3 -/0 1.9 V vs NHE), the anthraquinone derivatives were unable to effect repair [152]. We attribute the lack of repair by these anthraquinone derivatives to their particularly short-lived singlet states anthraquinone derivatives that do not rapidly interconvert to the excited triplet state are indeed effective at thymine dimer repair [151]. These observations suggest that interaction of the dimer with the singlet state may be essential for repair. 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

A number of papers have reported studies on pyrimidine radical cations. 1-Methylthymine radical cations generated via a triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid were detected by Fourier transform electron paramagnetic resonance (FTEPR). The parent 1-methylthymine radical cation, and its transformation to the N(3)-deprotonated radical cation, were observed. Radical cations formed by addition of HO and POs" at C(6) were also detected depending on the pH. Similarly, pyrimidine radical cations deprotonated at N(l) and C(5)-OH were detected from the reaction of 804 with various methylated pyrimidines." These radicals are derived from the initial SO4 adducts of the pyrimidines. Radical cations of methylated uracils and thymines, generated by electron transfer to parent ions of... 

Closer examination of the mechanism of photochromic transformations in phenoxynaphthacenequinones by pica- (25 ps) and nanosecond (8 ns) photoexcitation showed that photoisomerization of these compounds, as in photochromic anthraquinones, is an adiabatic reaction that proceeds through the triplet state of the initial form 54 ... 

The photolysis of benzoyl azide can be sensitized efficiently by benzophenone" . Other conventional sensitizers such as naphthalene, triphenylene and anthraquinone have been reported to have only a small effect on the photoreaction, diazo-iso-butyronitrile and fluo-rene to have none. In the benzophenone-sensidzed reaction, where triplet nitrenes are formed directly, the only reaction product in alcohol soludon is benzamide, which is obtained in quandtadve yield. This demonstrates again the different chemistry of singlet and triplet nitrene routes in azide photolysis. 

Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(II) tris(2,2 -bipyridyl) complexes have been synthesized bearing appended anthraquinone or tyrosine functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(III) tris(2,2 -bipyridyl) complex... 

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